1. Field of the Invention
The invention pertains to the field of silent chains. More particularly, the invention pertains to a silent chain with increased strength and stiffness.
2. Description of Related Art
Silent chains or inverted tooth chains have been used for many years for power transmission applications, especially in automotive powertrains. In automatic transmissions, inverted tooth power transmission chains can be found as input drives between the torque converter and input gear set, or as an output drive between the output gear and the final drive. Inverted tooth chains can also be found in transfer case applications between the input of the transfer case and the output shaft to the front axle. A special class of inverted tooth chain that uses pin and rocker joints to achieve articulation of the chain joint are used where very low losses are required from friction and wear. Most conventional inverted tooth chains use a simple round pin fitted to a round hole in the link to articulate the chain by the link sliding on the round pin. This sliding produces greater friction than the pin and rocker design mentioned above where the pin and rocker can roll on one another.
Inverted tooth chain is limited in the tension that it can transmit, either through metal fatigue of the chain links causing the chain to fail, or by the chain jumping teeth on a sprocket which causes a momentary loss of the ability to transmit tension between the two sprockets. As the chain wears, slack is built up in the drive which reduces the ability of the chain to transmit tension without jumping teeth. A successful chain design balances fatigue strength, the ability to resist tooth jump, chain wear and operating noise level. In trying to improve one characteristic, another one is often compromised.
At the same time, vehicle manufacturers are fitting transmissions with more gears to their vehicles to improve performance as well as reduce fuel consumption. The wider overall transmission ratio increases the chain tensions and makes tooth jump by the chain more likely to occur under severe maneuvers. This is heightened by application of brake traction control to prevent the wheels from spinning, which relieves overload conditions from occurring within the drivetrain. At the same time, the wider ratio transmissions reduce engine speed during highway cruising which improves fuel economy. The lower engine speed reduces the masking noise making chain noise more audible in the passenger compartment of the vehicle. The result is the need for an improved chain that provides significantly better tooth jump resistance while still providing low friction losses in operation along with lower operating noise when used as an output drive in an automatic transmission or as a transfer drive in a transfer case.
Power transmission chains of the inverted tooth design for automotive use have relied on a link profile with an included angle of the outer flanks of 60 degrees. This produces an angle of 30 degrees to a vertical line intersecting the chain flank. Initially the chains were designed to engage the sprockets on the outside flanks of the links and seat on the sprocket teeth on the outside flanks. Traditional silent chains used for timing chain applications have been designed to engage the sprocket teeth on the inner flank of the links but transitioned to the outside flanks as they fully articulated to wrap the sprockets. Most of these designs had an included angle of the outer flanks of 60 degrees although some silent chains used an included angle of 55 degrees.
These silent chains achieved a low operating noise level by having a long active flank for gradual engagement with the sprocket. This required a high crotch above the pitch line of the links. The result was a relatively weak link, since tension applied to the links through the apertures resulted in a high bending stress in the crotch of the links.
Attempts have been made to improve the chain strength by dropping the crotch of the links in respect to the pitch line of the chain to reduce the bending stress for a given applied tension. However, as the crotch was dropped, the length of active flank available for engagement with the sprocket was reduced. This resulted in an increase in noise during operation of the chain. This also resulted in shorter sprocket teeth which reduced the resistance of the chain to jumping teeth under certain operating conditions.
Prior art silent chains have limited strength, stiffness, and small jump torque values due to link design features, including but not limited to crotch height, included angle, and effective flank angle. There have been some attempts to decrease the included angle, effective flank angle, and the crotch height, however while this resulted in increased strength, it also resulted in a significant increase in undesired chain noise. Another problem with silent chains is wear on both the chain and the sprocket.
U.S. Pat. No. 5,236,400 discloses a silent chain where the engaging surface angles of the link plates are varied, with irregular links having different interengaging surface links in reference to a standard link plate being irregularly arranged in the longitudinal direction of the chain. The engaging location of the link plates with the engaging surface of the involuted sprocket teeth are dispersed over an entire region of the engaging surfaces of the sprocket teeth, to avoid a concentration of wearing at one specific location. The engaging position of the irregular links is not concentrated at a specific location, but instead the engaging surface angles of the link are made different. The irregular shaped link has an engaging surface angle of (α+Δα), which is larger than the engaging angle α of the standard link. The included angle for the standard link is 60 degrees and the included angle of the irregular links is 80 degrees. The flank angle for the standard link is 30 degrees and the flank angle of the irregular links is 40 degrees.
U.S. Pat. No. 5,267,910 discloses a silent chain where each of the link plates has meshing surfaces adapted to engage the teeth of a sprocket at a pitch line. The majority of the link plates have a modified tooth profile in the form of a continuous curved surface composed of a concave circular arc having a radius of curvature, where the convex arc is positioned to interfere slightly with the sprocket teeth which it engages. The center of curvature of the convex arc is situated on the dedendum side of the pitch link relative to the chain. The included angle is 60 degrees and the flank angle is 30 degrees.
U.S. Pat. No. 6,244,983 discloses links of a silent chain that engage sprocket teeth on their inside flanks during initial engagement and full engagement. The sprocket teeth have a flank shape with a first part that matches the lower portion of the shape of the inside flank of the link tooth and a sprocket tooth flank shape with a second part that matches the upper part of the link tooth shape.
U.S. Pat. No. 6,334,828 discloses link plate teeth with inner and outer flanks profiled to satisfy the expression Hi=Ho+Hs, where Hi is the distance from the chain pitch line to an inner flank pitch line, Ho is the distance from the chain pitch line to the outer flank pitch line, and Hs, is the amplitude of polygonal motion of the chain. The included angle of the links is 60 degrees. In prior art FIG. 5 of U.S. Pat. No. 6,334,828, Hi is less than or equal to Ho. The included angle of the prior art link is 65 degrees. The flank angle of the prior art link is 32.5 degrees.
Another problem with prior art links results from attempts to increase the strength of the links with an included angle of sixty degrees or a flank angle of thirty degrees while simultaneously reducing the crotch height. While the strength of the chain and the resulting stiffness has increased, the combination of the two above alterations have resulted in the generation of chain noise at an unacceptable level. Other prior art links attempt to vary the point of contact with the sprocket teeth on the link to reduce noise, but compromise by having the crotch so high that the apex of the crotch is either equal to or above the bottom of the pin apertures, decreasing the strength, stiffness, and tooth jump torque.
Similarly, U.S. Pat. No. 6,796,920 discloses the expression Hi=Ho+Hs, where Hi is the distance from a pin center line to a pitch line of the inside tooth faces, Ho is a distance from the pin center line to a pitch line of the outside tooth faces, and Hs is the amplitude of polygonal motion of the chain. In prior art FIG. 7 of U.S. Pat. No. 6,796,920, the link plate has a crotch that is higher than the bottom of the pin apertures and has an included angle of 56 degrees. The flank angle of the link is 28 degrees.
JP Patent Application No. 60-164042 discloses a silent chain with three different types of link plates. Each of the links have a different pressure angle and are irregularly arranged along the chain to prevent improper tooth mesh due to tooth jumping. The crotch of the links are equal or higher than the bottom of the pin apertures. The included angles of the links are 55 degrees, 75 degrees, and 85 degrees. The flank angles of the links are 27.5 degrees, 37.5 degrees, and 42.5 degrees respectively.
FIG. 1 shows a prior art link 100 that contacts and seats on a sprocket on the outer flank 104 of the link. The link has a pair of pin apertures 102 for receiving connecting pins comprised of a rocker pin 107 and a second pin 108 and a pair of teeth 105. The teeth 105 have outside flanks 104 and inside flanks 106 that form the crotch 103 of the link. The pitch (P) of the link is the average distance between the joints or apertures 102 of the link when it is in an assembled chain. The pitch line is defined as the pin and rocker contact point when the link is assembled in a chain that is in a straight line approaching a sprocket. A variable K shows the measured distance between the pitch line and a line which contacts the outside flank at 1.5*Pitch (1.5 P). Variable H is the measured distance below the pitch line to the apex of the crotch 103 of the link. A variable T shows the measured distance from the pitch line down to the tip of the tooth on the link, shown as the toe 105 of the link.
On the outer flanks 104 of the link, Fo shows the contact area on the outer flank 104 in which the link 100 contacts or seats on a sprocket tooth. The effective outer flank angle Δ is defined as the angle formed by a line tangent to the outer flank of the link at the 1.5 P line and a vertical line intersecting the tangent line. The prior art link 100 has an effective outer flank angle Δ of thirty degrees.
If the link has only a single round pin (not a rocker pin), the pitch line for the chain would pass through the center of the single round pin.
FIG. 2 shows a prior art link 110 that contacts the sprocket on inside flank 116 of the link and seats on the outer flank 114 of the link. The link has a pair of pin apertures 112 for connecting pins comprised of a rocker pin 117 and a second pin 118 and a pair of teeth 115. The teeth 115 have outside flanks 114 and inside flanks 116 that form the crotch 113 of the link. The pitch (P) of the link is the distance between the contact points of the rocker pin 117 and the second pin 118 forming the rocker joint in apertures 112 of the link, in an assembled chain. The pitch line is defined as the line passing through pin and rocker contact point when the link is assembled in a chain that is in a straight line. A variable K shows the measured distance between the pitch line and a line which contacts the outer flank at 1.5*Pitch (1.5 P). The prior art link 110 has an effective outer flank angle Δ of thirty degrees. The effective outer flank angle Δ is defined as the angle formed by a line tangent to the outer flank of the link at the 1.5 P line and a vertical line intersecting the tangent line.
A variable L shows the measured distance between the pitch line and a line which contacts the inner flank at 0.5*Pitch (0.5 P). Variable H is the measured distance below the pitch line to the apex of the crotch 113 of the link. A variable T shows the measured distance from the pitch line down to the tip of the tooth on the link, shown as the toe 115 of the link.
On the inner flanks 116 of the link, Fi shows the contact area on the inner link flank 116 in which the link 110 contacts a sprocket tooth. The effective inner flank angle φ is defined as the angle formed by a line tangent to a radius, forming the inside flank 116 and whose center is located outside the periphery of the link, at the contact point of the radius to a horizontal line spaced 0.5*P from one inside flank 116 to the other inside flank and a vertical line intersecting the tangent line. The prior art link 110 has an effective inner flank angle φ of thirty degrees.
Therefore, there is a need in the art for a new chain that has enhanced resistance to tooth jump that still provides low noise level in operation with enhanced fatigue strength with links that have increased strength, a low crotch, and small effective flank angle.